Melanin is the black pigment in hair and skin and is synthesized from amino acid tyrosine by melanosomes. Melanosomes are organelles found in melanocytes, a cell type present at the dermis-epidermis junction. Melanin plays an important role in protecting human body from the harmful effects of ultraviolet rays. Melanin is also important in medical science and cosmetology. The biosynthesis pathway of melanin involves the catalytic hydroxylation of tyrosine to L-3,4-dihydroxyphenylalanine (L-DOPA) by tyrosinase and the conversion of L-DOPA to dopachrome. Tyrosinase (EC 1.14.18.1) is a copper-containing monooxygenase that is widely distributed in nature. Its primary metabolic function is to catalyze the oxidative degradation of the amino acid tyrosine. This degradation takes slightly different routes in animals, plants and microbes, but the rate-controlling first steps—those catalyzed by tyrosinase—are the same in virtually all living species. In animals, including humans, tyrosinase first transforms tyrosine into 3,4-dihydroxyphenylalanine (DOPA), then to the corresponding quinone (DOPAquinone), and finally to 2-carboxy-2,3-dihydroindole-5,6-quinone (DOPAchrome), which is further converted by other enzymes to still more highly oxidized materials including the melanin substances responsible for skin pigmentation.
Excessive formation of melanin following prolonged sun exposure or due to disorders of epidermal melanin units is responsible for erythema, sunburn, melasma, ephelides, and pigmented cosmetic dermatitis. There have been several reports on use of inhibitors of tyrosinase such as hydroquinone and its derivatives, kojic acid, catechols, mercaptoamines, alpha hydroxy acids, etc., in cosmetic or pharmaceutical compositions to regulate skin pigmentation. Taketsugu Tadokoro indicates that tyrosinase is the most critical enzyme for synthesis of pigment, and its levels showed a marked response to UV (J Invest Dermatol 124:1326-1332, 2005). The correlation between melanoma and tyrosinase inhibition is accepted by the medical profession. Thus, development of agents capable of modulating the enzyme activity of tyrosinase will have considerable value in the control of the above-mentioned undesirable skin conditions (Hideya Ando et al., Journal of Investigative Dermatology (2007) 127, 751-761). The pharmaceutical, cosmetic, and food industries and the like all feel the need to develop agents for tyrosinase inhibition and for prevention and therapy of symptoms resulting from undesirable effects of tyrosinase activity.
Non-toxic natural products useful in the formulation of cosmetics and pharmaceuticals are of considerable interest. Firethorn (Pyracantha) is a genus of large thorny evergreen shrubs in the family Rosaceae, subfamily Maloideae. It is native to the area from southeast Europe to southeast Asia and closely related to Cotoneaster, but has serrated leaf margins and numerous thorns (Cotoneaster is thornless). A. Falodun et al. indicates that the presence of active principles in the leaf extracts of P. staudtii may be responsible for some of the remedies in traditional medicines for threatened abortion and dysmenorrheal (Pak J Pharm Sci. 2005 October; 18 (4):31-5). Otsuka H et al. discloses that Pyracantha crenulata roem has an anti-inflammatory effect (Chem Pharm Bull (Tokyo). 1981 November; 29(11):3099-104). CN 1765912 discloses an extract of Pyracantha fortuneana with water, methanol, ethanol, propanol, butanol, acetone or their mixtures which can remove oxygen free radicals. WO 0135971 discloses whitening compositions for oral administration which contain a solvent extract of a plant belonging to Rosaceae Pyracantha and utilization of the same, wherein the water extract and water/ethanol extract of Pyracantha fortuneana are effective in inhibiting tyrosinase. Japanese Publication No. 05-058870 provides an extract of Rosaceae Pyracantha with water, methanol, ethanol, propanol or their mixture which has beautifying and whitening effects. Furthermore, Jiang et al. finds that an extract of Pyracantha koidzumii has less cytotoxic and higher cellular tyrosinase inhibitory activity. Other studies indicate that Pyracantha koidzumii is effective in inhibiting tumors (Cancer Res. 1966; 26B(11):1302-453) and Pyracantha fortuneana has effects on blood coagulation (Zhong Yao Cai 2001; 24(12):874-6). The ingredients of Pyracantha including carotenoid, flavonoid, glycoside and sterol derivatives have been isolated from Pyracantha. In particular, Dai Y et al. finds tyrosinase-inhibiting biphenyl glycosides such as 3,3′-dihydroxy-5′-methoxy-(1,1′-biphenyl)-4-O-beta-d-glucoside, 4′-hydroxy-2,3′,5′-trimethoxy-(1,1′-biphenyl)-2′-O-beta-d-glucoside, 4′-hydroxy-3′,5′-dimethoxy-(1,1′-biphenyl)-2-O-beta-d-glucoside, 2,4′-dihydroxy-3′,5′-dimethoxy-(1,1′-biphenyl)-3-O-beta-d-glucoside, and 3,4′-dihydroxy-3′,5′-dimethoxy-(1,1′-biphenyl)-4-O-beta-d-glucoside (J. Nat. Prod. 2006; 69(7):1022-1024).
However, none of the Pyracantha koidzumii extracts provided by the above-mentioned prior art references shows superior tyrosinase inhibitory activity. There is still a need for tyrosinase inhibitors capable of effectively inhibiting the activity of tyrosine and which can be used on a long-term basis without undesirable side effects on the human skin.